"The delays in this programme mean that we have more than an entire squadron of attack helicopters sitting in sheds. They should be flying by now," he said.

'Reduced effectiveness'

The delay means dozens of aircraft will have to be stored away in hangars while the completion date for the initial training programme for 144 pilots is put back from April 2004 to February 2007.

The NAO warned the delay in training also put in jeopardy the delivery of 16 more Apache helicopters, scheduled for February 2005.

That would reduce the Army's capability, it added.

Contractual problems over the supply of spares could mean the Apaches kept in storage may have to be used for parts.

Training estimates were based on US figures

The initial order for 67 Apaches, placed with Westland Helicopters in 1995, was said to offer the greatest advance in Army effectiveness since the tank.

A separate PFI agreement was reached with ATIL - a company jointly-owned by Westland and the Apache's US manufacturer Boeing - to cover the training.

But delivery of the hi-tech flight simulator was delayed by 17 months, pushing the start date for pilot training from 2001 to September 2003.

A spokeswoman for Boeing told BBC News Online: "We recognise we could have performed better to ensure the simulator was fully operational at an earlier time and we have significantly increased resources to resolve the issue."

The length of the training courses was extended from 15 weeks to 26, because of the £45m helicopter's complexity and the UK's poor weather conditions for flying.

As a result, the NAO said a "large number" of Apaches - some estimates say more than half - will have to be stored away in hangars at RAF Shawbury in Shropshire at a cost of £6m.

Other problems

The head of the NAO, Comptroller and Auditor General Sir John Bourn, criticised the decision to split the training programme from the main contract.

He said: "It is disappointing that because of problems with the training programme the
helicopters are not expected to provide a brigade-level capability until February 2007."

The MoD said it had noted the NAO's concerns and steps were being taken to try to reduce the duration of the pilot training courses without
compromising the quality.

But there are other problems holding back the Apache fleet.

Both the US and the British have encountered problems when firing anti-tank rockets, including debris hitting parts of the aircraft.

The NAO has disclosed that the first Apache crews will have no secure radio communications with British ground troops or other helicopters - although they will be able to speak to the Americans - because of delays to the Army's Bowman radio system.